Gascogne is a province of France in the south west on the border to Spain. There was a lot of intermixing between people from all over Europe and the Mediterranean thanks to things like Crusades, Muslim States of Spain but especially trade. So yes, people can end up looking quite dark but as mentioned above no one would have batted an eye at that.
Don't worry too much about the Gascon language. It's a dialect of occitan, a roman language spoken in south france and north spain. Not too many people speak it nowadays and D'artagnan would have spoken French because he's nobility (at least in real life and in Dumas' books).